The present disclosure describes a method for forming high resolution patterns using nanoimprint lithography on top of or using chemically sensitive organic electronic materials. More specifically, this disclosure details a method for patterning fine metal lines on top of organic semiconductors for making high-density organic memory chips.
Nanoimprint lithography (NIL) and related hard mask patterning techniques such as Flash and Repeat Nanoimprint Lithography (S-FIL) or Jet and Flash Imprint Lithography (J-FIL) are thin-film patterning techniques that utilize a hard mask or “stamp” to impress a structure on an underlying polymer material known as a resist. Once the stamp is pressed into the resist and released, the pattern of raised areas on the stamp are replicated in the resist. The resist pattern can be transferred into the underlying material using an etch process before the resist is removed from the substrate using a stripping process, usually involving an oxygen plasma etch.
Imprint lithography is a very high-resolution technique because it doesn't suffer from the diffraction limitations of photolithography. The stamp can be made using a slower, high-resolution technique such as ebeam lithography, which limits the features to 10 nm or less, making it an attractive future lithography technique for high resolution patterning for memory and integrated circuits.
Liftoff is another process that is possible with imprint lithography, whereby a resist layer is imprinted, excess resist is removed with an etch process (descum), a material such as metal is deposited on the patterned substrate, and the resist is removed, leaving material in the “holes” of the original imprint pattern. This process is particularly good for patterning metals that do not have a favorable etch chemistry available, such as platinum.
This liftoff technique does not work well for patterning, or patterning on top of, chemically sensitive materials such as organic semiconductors. These materials, used in devices such as Organic Light Emitting Diodes (OLED), Organic Thin-Film Transistors (OTFT) or Organic Memory (OMEM), have need for high resolution patterning of both the semiconductors themselves (for example in high-resolution OLED pixels), or of other materials such as metals on top of these layers (e.g. source/drain electrodes for OTFTs and top electrodes for OMEM). The reasons for this limitation are the harsh solvents used for removing the resist during liftoff and the need for a descum step, which would limit the use of the liftoff process on top of the chemically sensitive materials.
Various devices are known that employ patterned organic materials aligned to features or structures on a substrate. These devices are often considered to have the potential for low cost since organic materials are typically less expensive than inorganic materials and these organic materials can be rapidly blanket coated over large substrates, permitting the formation of large, low cost devices. One example of such a device is a display employing Organic Light Emitting Diodes (OLEDs). Besides providing the potential for low cost, these displays have the potential to produce light much more efficiently and with higher visual quality than most competitive display technologies. Therefore, OLED displays have the potential to displace LCD and plasma displays in many markets. This OLED technology can additionally be employed in other devices, including lamps with adjustable color. Similar device structures can be employed using organic semiconductors to form organic photovoltaic devices, organic memory devices and organic electrical components; such as Organic Thin Film Transistors (OTFTs).
Unfortunately, OLED technology, particularly OLED display technology, has been adopted slowly. This slow adoption rate stems at least partially from the high cost of patterning these materials to form a practical display device. Various approaches to patterning organic materials to form full color OLED displays have been attempted. Patterning of different colors of material by vapor deposition of organic materials through shadow masks has proven to be effective. However these shadow masks limit the resolution of the displays, the size of the substrate that can be successfully coated, and increase the TACT time. Other approaches, such as the use of laser deposition to pattern color emitters has been demonstrated but this technology often produces displays with low yields and often results in significant residual waste. Solution printing of different colored organic emitters has also been discussed but these processes typically result in emitters with significantly lower emission efficiency as compared to emitters deposited by vapor deposition. This lower efficiency is due to increased contact resistance and the fact that polymeric materials, that are often used, often have a lower luminescent efficiency and lifetime than small molecule materials and the use of solution deposition limits the number of layers that can be deposited on one another to manage the movement of carriers through the organic layer. Other approaches to forming multicolor OLED devices have also been attempted, including the use of white emitters together with patterned color filters. However, these approaches also reduce the effective efficiency of the emitters within the OLED display. Other organic devices, including OTFTs, suffer from similar patterning issues.
One approach to avoid detailed patterning of organic materials is to adopt OLED display structures including one or blanket-coated emitting layers. For example, Miller et al in U.S. Pat. No. 7,142,179, entitled “OLED display device”, issued on Nov. 26, 2006 and Cok et al in U.S. Pat. No. 7,250,722, entitled “OLED device”, issued on Jul. 31, 2007, each discuss a structure having a first OLED constructed between a first and a second patterned electrode and a second OLED constructed between the second patterned electrode and a blanket-coated electrode. Within each of these documents, the first OLED must be patterned to permit the second pattern electrode to be connected to the substrate. Further the second electrode must be patterned after it is deposited over the OLED. These structures produce higher efficiency light output without patterning at least one of the layers of organic materials. However, these structures require the patterning of very small structures through an organic layer to form a via hole through the organic material, as well as the patterning of a conductive layer over an organic layer. Robust processes for providing these vias and forming the electrode pattern over an organic layer in a high speed manufacturing environment are not known in the art and therefore these device structures have not been successfully manufactured. Similar structures are also desirable for the formation of multi-layer photovoltaic and other organic devices.
In inorganic electronic devices, it is known to apply photolithographic techniques to pattern multiple thin film layers of inorganic semiconductors and inorganic electrically conductive layers with high resolution over large substrates for forming arrays of electrical components. Unfortunately, the photolithographic materials and solvents applied to form these devices are known to dissolve organic materials. Therefore, it is not possible to apply the photolithographic materials and solvents that are known to be used to manufacture inorganic solid state circuits to pattern layers of organic material, especially layers that include active semiconductor organic materials or layers that are formed on top of organic materials.
Recently photoresist materials and solvents have been discussed in the art to facilitate the use of photolithographic techniques to pattern polymeric organic semiconducting layers. For example, Zakhidov et al. in an article published in Advanced Materials in 2008 on pages 3481-3484 and entitled “Hydrofluoroethers as Orthogonal Solvents for the Chemical Processing of Organic Electronic Materials” discusses a method for patterning polymer organic material in which a fluorinated photoresist is deposited on a substrate, selectively exposed to an energy source to change the solubility of a portion of the photoresist, the photoresist is developed in a solvent including hydrofluroether to develop the pattern and remove the portion of the photoresist material that was not exposed. The solubility of the cross-linked photoresist in a hydrofluroether was then reestablished through the use of another solvent. An active organic semiconductor was then deposited over the remaining photoresist and remaining photoresist was lifted off to pattern the active organic semiconductor. As such, this paper demonstrates the patterning of a single solution-coated, polymeric, organic semiconductor on a substrate. The same general process has been discussed by Lee et al. in an article published in the Journal of the American Chemical Society in 2008 on pages 11564 through 11565 and entitled “Acid-Sensitive Semiperfluoroalkyl Resorcinarene: An Imaging Material for Organic Electronics”.
Taylor et al. in an article published in Advanced Materials on Mar. 19, 2009 on pages 2314-2317 and entitled “Orthogonal Patterning of PEDOT:PSS for Organic Electronics using Hydrofluoroether Solvents” discusses the formation of a bottom contact thin film transistor in which a polymeric organic conductor (i.e., PEDOT:PSS) is formed on a substrate, a photoresist is formed and patterned over the conductor, the conductor is etched, a second photoresist is applied and patterned before an organic semiconductor (e.g., pentacene) is applied and patterned.
Each of these papers discuss patterning of solution-coated, polymeric organic materials using a modified photolithographic process and materials to create components in an electrical circuit, the use of processes and materials such as these have not been applied to OLED devices. Further, these papers discuss the application of these materials and processes for use with polymers and do not provide a method for patterning layers of small molecule organic materials. Further, according to this method, it is necessary to perform multiple photo-patterning steps, specifically one photo-patterning step for each patterned layer, to create patterns in multiple layers including at least one organic layer and a layer deposited over this organic layer, such as an electrical conductor. Certain photolithographic process steps, including the exposure of the photolithographic materials to radiation to perform the photo-patterning step, are typically performed in air. Unfortunately, air contains oxygen and moisture with which the organic materials can react. Therefore, performing multilayer photo-patterning of organic devices by forming multiple layers of photo-patternable materials, some being formed over the organic layers can result in devices with degraded performance. Further, each of these photolithographic steps are quite expensive to perform and require one full photolithographic step, including deposition of the photo-patternable material, as well as, exposure, development and liftoff of the pattern for each layer within the device.
In another approach, Katz and Dhar in International Publication Number WO 2009/126916, entitled “Patterning devices using fluorinated compounds” filed Apr. 10, 2009 discuss forming an active layer to be patterned on a substrate, providing a barrier layer of fluorinated material over the active layer, forming a photo-patternable layer over the fluorinated layer and exposing the photo-patternable to radiation within a process that permits the active layer to be patterned. However, this approach requires the deposition of multiple layers to pattern a single active layer.
In another approach, Taussig et al in U.S. Pat. No. 7,202,179 entitled “Method of forming at least one thin film device”, issued on Apr. 10, 2007 discusses a method in which a 3D template is used to emboss a 3D structure over the layers of a device and the 3D structure and the underlying layers are etched to provide the final structure. This method permits different patterns to be formed in different layers of a device as the result of using a single imprint lithographic step so that alignment of multiple lithographic steps is not required, permitting this method to be applied to form devices on substrates that are not stable, for example, plastics which expand or contract during manufacturing. Unfortunately, the method provided is only useful with inorganic structures as the method applies materials and methods that are not compatible with organic semiconductor materials. Further, the method applies techniques that are not currently used in high volume manufacturing and requires strict process control to permit the patterning steps to achieve the desired result.
There is, therefore a need for a method that permits the formation of a first pattern in an organic material layer and at least a second, different pattern in a separate active material layer formed over the organic layer. This process should be robust, permit the formation of patterns at near micron resolution and not require the organic materials to be exposed to air during development. It is especially desirable that this method be compatible with vapor deposited, small molecule organic materials. Ideally, this same method would also be compatible with non-organic devices to permit multiple layers within these devices to be differently patterned in response to a single photolithographic step.